For producing high-grade metals and metal alloys, which have the least possible inclusions of foreign objects and are homogeneous in structure, various methods are known. One of the best-known methods is electric-arc melting, in which an electrode extends towards a crucible and, by applying an electric potential between the electrode and the crucible, the tip of the electrode melts away and falls as a liquid material into the crucible. As a rule, the so-called melting electrode is connected to one pole of a direct-current voltage and the crucible to the other pole of this direct-current voltage. However, superpositions of alternating-current voltages are also possible to achieve particular effects.
A major problem in operating arc melting furnaces of the above-mentioned type lies in the control of the arc length, that is, of the distance between the lower end of the electrode and the surface of the melted material that is already in the crucible. If the arc is too long, the electrode and/or the melted material could be heated wrongly, so that the quality of the melted material is greatly reduced. Since, on the one hand, the level of the melted material in the crucible is constantly rising and, on the other, the distance between the end of the electrode and the surface of the crucible cannot be observed directly, special measures must be taken to control this distance.
In arc furnaces which operate at or only slightly below atmospheric pressure, the arc length is controlled by maintaining a given arc voltage. At atmospheric pressure, the plasma is characterized by the fact that it has a particular voltage gradient, for example, 20 volts per 2.5 cm. The voltage drops at the cathode and anode surfaces together amount to an additional 20 volts, so that, if an arc length of, for example, 1.25 cm is to be maintained, the electrodes will then have to be brought into such a position that the arc voltage is 30 V. This can be realized by means of conventional equipment which measures and controls the arc voltage.
In arc melting furnaces which operate under vacuum, the above-described method can, however, not always be employed. Such arc melting furnaces are used especially for melting so-called refractory active metals, such as titanium or zirconium, as well as for preparing stainless steels and high-temperature alloys. When the gas pressure, which surrounds the arc, decreases, the voltage gradient of the arc plasma also decreases and, at very low pressures, the voltage gradient of the arc plasma may, for example, by only one volt per 2.5 cm. Since the anode and cathode voltage drops for steel, for example, are approximately 20 volts, the voltage drop at the arc is very small in comparison with the remaining voltage drops. Changes in the gas content and the alloy composition affect the anode and cathode voltage drops in the order of magnitude of the voltage drop in the "arc column". Consequently, the method of controlling the arc by keeping the arc voltage constant is not very effective, especially in the case of steels, since the actual length of the arc will usually deviate greatly from the desired length.
An arc melting furnace has meanwhile become known which, for controlling the distance between the electrode and the surface of the melted material, makes use of the knowledge that, even in normal operation, the voltage breaks down briefly at certain time intervals (U.S. Pat. No. 2,942,045). This effect is caused by short-circuits, which originate from liquid metal droplets, which drop from the electrode into the crucible and briefly connect the electrode electrically with the melted material in the crucible. As long as the duration and frequency of these short-circuits are not very large, the arc operates almost at full power, so that there is no substantial effect on the heating of the melted material. If the arc becomes shorter, the frequency of the arc short-circuits increases.
In accordance with the known arc melting furnace, the distance between the electrodes is controlled by maintaining the frequency of the arc short-circuits within a particular range. For example, a voltmeter is observed and the time intervals between the individual droplets are measured with a stopwatch, so that the droplets per unit time can be determined.
Other known equipment for controlling the distance between the electrodes in an arc melting furnace is premised on the knowledge that voltage fluctuations in the form of positively increasing pulses, each of which occurs for a short period of, for example, 40 milliseconds with a frequency of 30 Hz, are superimposed on the arc voltage (German Pat. No. 1,212,651). These voltage pulses, the cause of which need not be discussed herein, are used to control the electrode distance by dividing the voltage curve into a basic component and a second component. The pulse-shaped fluctuations, which occur in the second component as voltage, current or impedance fluctuations, are detected and the distance between the electrodes is controlled as a function of the repetition frequency of these fluctuations. A pulse count of the overvoltages per unit time is thus carried out and, if the number of pulses is too low, the electrode is lowered.
With other known equipment for arc melting, the starting points are oscilloscopic or oscillographic observations which show that brief short-circuits of 0.1 to 0.3 seconds occur between the electrode and the molten metal surface of the crucible during the melting of metals in vacuum. In addition, it is taken into consideration that changes occur in the arc voltage which arise from impurities which, in turn, are based on changes in the composition or in the pressure of the inert gas atmosphere or are caused by a deflection of an arc from the electrode to the crucible wall, these last-mentioned voltage changes being less than the voltage changes which occur when droplets of molten metal connect the electrode with the molten metal bath (U.S. Pat. No. 2,915,572). This known arrangement includes equipment with which the electrode is moved in the direction of the surface of the metal in the crucible by an amount which is at least equal to the difference between the melting rate and the rate at which the metal surface rises. The arrangement furthermore has equipment which, on the basis of the molten droplets between the electrode and the metal surface, is activated in a specified position of the electrode in relation to the surface of the metal in order to move the electrode a certain distance away from the metal surface. The voltage short-circuits are detected here by a relay, which controls a timer.
Furthermore, an arrangement for controlling the distance between electrodes is known in which the droplet short-circuits between the electrode and the liquid metal surface of the crucible are used as a control criterion (U.S. Pat. No. 4,578,795). The droplet short-circuits and the corresponding voltage reductions appear here as iterative pulses, which are closely correlated with the distance between the electrodes. The number of droplet short-circuits is summed and, every time that the number of short-circuits has reached a specified value, the average period between the short-circuits, as well as the time required to reach this value, are calculated and stored. A microprocessor is used to make these calculations and to display the duration of each short-circuit. Immediately after the pulse shaping of the natural droplet short-circuits, the system operates digitally. The normalized pulses are supplied to an event counter. The pulse quantity is previously entered manually and can be changed from case to case. If the contents of the event counter fall below the pulse count that has been set, as can be determined by coincidence, a command is given to the timer, and the time that has elapsed between the respective coincidences is read. This value serves as a measure of the distance between the electrode and the liquid metal surface. The reading is renewed whenever the given number of droplet short-circuits is reached (e.g., approximately 100 short-circuits).
An arrangement of this type has two disadvantages; namely, that the reading is renewed only after relatively long intervals of time and thus is not current, and that when there are few or even no drops, the time that elapses before a control intervention becomes very long. On the other hand, if the droplet count is high, the control intervention is very rapid. The droplet count determination times represent a dead time. This dead time is differently long for different operating conditions. The time response of the metering element is nonlinear. The phase rotation of the signal thus depends on the momentary operating state. The magnitude of the control intervention, when there is a deviation from the desired value, must be severely limited to avoid oscillations. The circuit amplification must thus be small. This brings about a sluggish control of "disturbance variables" with large deviations from the desired value.
Finally, yet another method and an associated arrangement are known for controlling the electrode drive velocity in an arc furnace. In this case the time between two consecutive droplet short-circuits is measured and the average time between a specified number of previous short-circuits is calculated (U.S. Pat. No. 4,303,797). For example, the time interval between the ten last short-circuits is calculated and supplied as an actual value to a controller. The transitional behavior of this equipment in the event of signal changes is less than advantageous.